It is highly desirable that ink and overprint formulations utilized for graphic art applications possess certain high performance characteristics. As typical surfactant-based emulsion polymerization products do not have the desired rheology properties for such applications, it is common practice to add a support resin in place of, or in addition to, the surfactants. These water and alkaline soluble (at pHs of about 8) support resins are added to the emulsion polymerization reaction to improve the stability and theological properties of the latex. Improved stability results from absorption of the support resin on the surface of the particles and the increased viscosity of the emulsion.
The traditional processes for producing polymeric support resins are well-known (see generally U.S. Pat. Nos. 4,839,413 and 5,216,064, which are hereby incorporated by reference). Commonly a solution polymerization reaction is employed wherein styrenic monomer and acrylic acid is mixed with a hydrocarbon solvent, a polymerization initiator, and a chain transfer agent. Upon completion of the reaction, the solution is stripped of the solvent to yield the acrylic polymer. The polymer is then available for use as a support resin when dissolved in an ammoniacal water solution.
However, problems exist with these traditional methods of producing support resins. For example, these methods require the use of environmentally adverse hydrocarbon solvents. Moreover, as these solvents are not usable or desirable in water-based ink or overprint formulations, the solvents must be stripped from the resulting acrylic polymers (thereby causing a yield loss). This stripping step also adds expense to the process due to both the loss of yield and the energy consumed in performing the stripping.
U.S. Pat. No. 6,172,149, which is incorporated herein by reference, teaches the preparation of rosin-fatty acid vinylic polymer sizing compositions via a polymerization method which does not require the use of hydrocarbon solvents. However, a problem exists with the employment of such polymers as support resins for emulsion polymerizations in that the resultant polymer emulsions are relatively low in viscosity when compared to similar latices made with acrylic support resins produced by conventional techniques. These low viscosities render the latices unsuitable for certain applications, such as architectural paints and flexographic inks. Another shortcoming is that the use of support resins based on unsaturated fatty acids yields latices with relatively high levels of residual monomers. These high levels of residuals can cause a latex to have a strong odor that most potential users of the latex find objectionable. A further disadvantage inherent in the use of both conventional acrylic support resins and the polymers taught in U.S. Pat. No. 6,172,149 lies in the fact that inks and coatings based on latices made with these resins commonly exhibit relatively low resistances to commercial cleaning formulations containing glycol ethers (such as FORMULA 409 available from the Clorox Co.) or alkaline chemicals. Resistance to these types of commercial cleaning formulations is highly desired, especially in inks employed in the printing of consumer packaging materials.
Therefore, an object of this invention is to solve these major problems by disclosing a method of producing carboxylic acid-modified vinylic polymeric compositions.
Another object of this invention is to disclose carboxylic acid-modified vinylic polymeric compositions which exhibit properties that make them useful as support resins in water-based graphic art ink and coating applications.